The Access Point operating in 802.11ax can use the Resource Units that were discussed in the previous article <Resource Units in 802.11ax> to setup packet send to multiple non-AP stations in a single transmission.

The Access Point assigns specific Resource Units to Specific stations. The AP encodes a packet meant for a specific station to those resource units to which it has been assigned to.

As it has been discussed earlier, the Access Point can use the MU-RTS trigger frame to block the channel. The MU-RTS is sent across the entire 20 MHz channel using OFDM so that all legacy clients can also look at the duration field and update their NAV. The MU-RTS is also used to synchronize the different clients who would be part of DL OFDMA reception. The synchronization is received since all the different clients need to send the Clear To Send (CTS) frame to the Access Point simultaneously.

The RU allocation for the different stations can be already negotiated via a Trigger frame earlier. The below figure provides an example for downlink transmission to two stations (STA1 and STA2) using DL-OFDMA mechanism.

UpLink Multi-User OFDMA in 802.11ax